Process for the production of L-amino acids using strains of the family enterobacteriaceae that contain an attenuated aceA gene

ABSTRACT

A process for the production of L-amino acids, in particular L-threonine, in which the following steps are carried out:  
     (a) fermentation of the microorganisms of the family Enterobacteriaceae producing the desired L-amino acid, in which the aceA gene or nucleotide sequences coding therefor are attenuated, in particular are switched off,  
     (b) enrichment of the L-amino acid in the medium or in the cells of the bacteria, and  
     (c) isolation of the L-amino acid.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Application Serial No. 60/283,384, filed Apr. 13, 2001, the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a process for the enzymatic production of L-amino acids, in particular L-threonine, using strains of the family Enterobacteriaceae in which the aceA gene is attenuated.

DESCRIPTION OF THE BACKGROUND

[0003] L-amino acids, in particular L-threonine, are used in human medicine and in the pharmaceutical industry, in the foodstuffs industry, and most especially in animal nutrition. It is known to produce L-amino acids by fermentation of strains of Enterobacteriaceae, in particular Escherichia coli (E. coli) and Serratia marcescens. On account of their great importance efforts are constantly being made to improve processes for producing the latter. Process improvements may relate to fermentation technology measures, such as for example stirring and provision of oxygen, or the composition of the nutrient media, such as for example the sugar concentration during the fermentation, or the working-up to the product form, for example by ion exchange chromatography, or the intrinsic performance properties of the microorganism itself.

[0004] Methods comprising mutagenesis, selection and mutant choice are employed in order to improve the performance properties of these microorganisms. In this way strains are obtained that are resistant to antimetabolites, such as for example the threonine analogue a-amino-β-hydroxyvaleric acid (AHV) or are auxotrophic for regulatorily important metabolites, and that produce L-amino acids such as for example L-threonine.

[0005] Methods of recombinant DNA technology have also been used for some years in order to improve strains of the family Enterobacteriaceae producing L-amino acids, by amplifying individual amino acid biosynthesis genes and investigating their effect on production.

SUMMARY OF THE INVENTION

[0006] The object of the invention is to provide new measures for the improved enzymatic production of L-amino acids, in particular L-threonine.

[0007] The present invention is based on the discovery microorganisms of the family Enterobacteriaceae which naturally produce L-amino acids do so more effectively under conditions in which the nucleotide sequence coding for the aceA gene is attenuated.

[0008] Thus, the object of the present invention may be accomplished with a process for the production of an L-amino acid, comprising:

[0009] (a) fermenting a microorganism of the family Enterobacteriaceae which produces the desired L-amino acid, in which the aceA gene or nucleotide sequences coding therefor are attenuated, in a medium;

[0010] (b) enriching the medium or the cells of the microorganism in the L-amino acid, and

[0011] (c) isolating the L-amino acid.

[0012] A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to FIG. 1 and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1: pMAK705ΔaceA (=pMAK705deltaaceA)

[0014] Length data are given as approximate values. The abbreviations and acronyms used have the following meanings: cat: chloramphenicol resistance gene rep-ts: temperature-sensitive replication region of the plasmid pSC101 ′aceB: part of the 3′ region of the aceB gene aceA′: ATG start codon of the aceA gene aceK′: part of the 5′ region of the aceK gene The abbreviations for the restriction enzymes have the following meanings: BamHI: restriction endonuclease from Bacillus amyloliquefaciens BglII: restriction endonuclease from Bacillus globigii ClaI: restriction endonuclease from Caryphanon latum EcoRI: restriction endonuclease from Escherichia coli EcoRV: restriction endonuclease from Escherichia coli HindIII: restriction endonuclease from Haemophilus influenzae KpnI: restriction endonuclease from Klebsiella pneumoniae PstI: restriction endonuclease from Providencia stuartii PvuI: restriction endonuclease from Proteus vulgaris SacI: restriction endonuclease from Streptomyces achromogenes SalI: restriction endonuclease from Streptomyces albus SmaI: restriction endonuclease from Serratia marcescens SphI: restriction endonuclease from Streptomyces phaeochromogenes SspI: restriction endonuclease from Sphaerotilus species XbaI: restriction endonuclease from Xanthomonas badrii XhoI: restriction endonuclease from Xanthomonas holcicola

DETAILED DESCRIPTION OF THE INVENTION

[0015] Where L-amino acids or amino acids are mentioned hereinafter, this is understood to mean one or more amino acids including their salts, selected from the group comprising L-asparagine, L-threonine, L-serine, L-glutamate, L-glycine, L-alanine, L-cysteine, L-valine, L-methionine, L-isoleucine, L-leucine, L-tyrosine, L-phenylalanine, L-histidine, L-lysine, L-tryptophan and L-arginine. L-threonine is particularly preferred.

[0016] The term “attenuation” describes in this connection the reduction or switching off of the intracellular activity of one or more enzymes (proteins) in a microorganism that are coded by the corresponding DNA, by using for example a weak promoter or a gene or allele that codes for a corresponding enzyme with a low activity and/or that inactivates the corresponding enzyme (protein) or gene, and optionally combining these measures.

[0017] By means of these attenuation measures the activity or concentration of the corresponding protein is generally reduced to 0 to 75%, 0 to 50%, 0 to 25%, 0 to 10% or 0 to 5% of the activity or concentration of the wild type protein, or the activity or concentration of the protein in the initial microorganism.

[0018] The process is characterized in that the following steps are carried out:

[0019] (a) fermentation of microorganisms of the family Enterobacteriaceae in which the aceA gene is attenuated,

[0020] (b) enrichment of the corresponding L-amino acid in the medium or in the cells of the microorganisms of the family Enterobacteriaceae, and

[0021] (c) isolation of the desired L-amino acid, in which optionally constituents of the fermentation broth and/or the biomass in its entirety or portions thereof remain in the product.

[0022] The microorganisms that are the subject of the present invention can produce L-amino acids from glucose, sucrose, lactose, fructose, maltose, molasses, optionally starch, optionally cellulose or from glycerol and ethanol. The microorganisms are members of the family Enterobacteriaceae selected from the genera Escherichia, Erwinia, Providencia and Serratia. The genera Escherichia and Serratia are preferred. In the case of the genus Escherichia the species Escherichia coli may in particular be mentioned, and in the case of the genus Serratia the species Serratia marcescens may in particular be mentioned.

[0023] Suitable strains of the genus Escherichia, in particular those of the species Escherichia coli, that produce in particular L-threonine, include for example:

[0024]Escherichia coli TF427

[0025]Escherichia coli H4578

[0026]Escherichia coli KY10935

[0027]Escherichia coli VNIIgenetika MG442

[0028]Escherichia coli VNIIgenetika M1

[0029]Escherichia coli VNIIgenetika 472T23

[0030]Escherichia coli BKIIM B-3996

[0031]Escherichia coli kat 13

[0032]Escherichia coli KCCM-10132

[0033] Suitable strains of the genus Serratia, in particular of the species Serratia marcescens, that produce L-threonine include for example:

[0034]Serratia marcescens HNr21

[0035]Serratia marcescens TLr156

[0036]Serratia marcescens T2000

[0037] Strains of the family of Enterobacteriaceae producing L-threonine preferably have, inter alia, one or more of the genetic or phenotype features selected from the following group: resistance to a-amino-β-hydroxyvaleric acid, resistance to thialysine, resistance to ethionine, resistance to a-methylserine, resistance to diaminosuccinic acid, resistance to a-aminobutyric acid, resistance to borrelidin, resistance to rifampicin, resistance to valine analogues such as for example valine hydroxamate, resistance to purine analogues such as for example 6-dimethylaminopurine, need for L-methionine, optionally partial and compensable need for L-isoleucine, need for meso-diaminopimelic acid, auxotrophy with regard to threonine-containing dipeptides, resistance to L-threonine, resistance to L-homoserine, resistance to L-lysine, resistance to L-methionine, resistance to L-glutamic acid, resistance to L-aspartate, resistance to L-leucine, resistance to L-phenylalanine, resistance to L-serine, resistance to L-cysteine, resistance to L-valine, sensitivity to fluoropyruvate, defective threonine dehydrogenase, optionally ability to utilize sucrose, enhancement of the threonine operon, enhancement of homoserine dehydrogenase, I-aspartate kinase I, preferably of the feedback-resistant form, enhancement of homoserine kinase, enhancement of threonine synthase, enhancement of aspartate kinase, optionally of the feedback-resistant form, enhancement of aspartate semialdehyde dehydrogenase, enhancement of phosphoenol pyruvate carboxylase, optionally of the feedback-resistant form, enhancement of phosphoenol pyruvate synthase, enhancement of transhydrogenase, enhancement of the RhtB gene product, enhancement of the RhtC gene product, enhancement of the YfiK gene product, enhancement of a pyruvate carboxylase, and attenuation of acetic acid formation. It has now been found that microorganisms of the family Enterobacteriaceae after attenuation, in particular after switching off the aceA gene, produce L-amino acids, in particular L-threonine, in an improved way.

[0038] The nucleotide sequences of the Escherichia coli genes belong to the prior art and may also be obtained from the genome sequence of Escherichia coli published by Blattner et al. (Science 277, 1453-1462 (1997)).

[0039] The aceA gene is described inter alia by the following data: Designation: Isocitrate lyase EC-No.: 4.1.3.1 Reference: Matsuoko and McFadden; Journal of Bacteriology 170, 4528-4536 (1988) Accession No.: AE000474

[0040] Apart from the described aceA gene, alleles of the gene may be used that result from the degeneracy of the genetic code or from functionally neutral sense mutations, the activity of the protein not being substantially altered.

[0041] In order to achieve an attenuation the expression of the gene or the catalytic properties of the enzyme proteins may for example be reduced or switched off. Optionally both measures may be combined.

[0042] The gene expression may be reduced by suitable culture conditions, by genetic alteration (mutation) of the signal structures of the gene expression, or also by antisense-RNA techniques. Signal structures of the gene expression are for example repressor genes, activator genes, operators, promoters, attenuators, ribosome-binding sites, the start codon and terminators. One person skilled in the art may find relevant information in, inter alia, articles by Jensen and Hammer (Biotechnology and Bioengineering 58: 191-195 (1998)), by Carrier and Keasling (Biotechnology Progress 15, 58-64 (1999), Franch and Gerdes (Current Opinion in Microbiology 3, 159-164 (2000)) and in known textbooks of genetics and molecular biology, such as for example the textbook by Knippers (“Molekulare Genetik”, 6^(th) Edition, Georg Thieme Verlag, Stuttgart, Germany, 1995) or that by Winnacker (“Gene and Klone”, VCH Verlagsgesellschaft, Weinheim, Germany, 1990).

[0043] Mutations that lead to a change or reduction of the catalytic properties of enzyme proteins are known from the prior art. As examples there may be mentioned the work by Qiu and Goodman (Journal of Biological Chemistry 272: 8611-8617 (1997)), Yano et al. (Proceedings of the National Academy of Sciences, USA 95, 5511-5515 (1998), Wente and Schachmann (Journal of Biological Chemistry 266, 20833-20839 (1991). Detailed information may be obtained from known textbooks on genetics and molecular biology, such as for example that by Hagemann (“Allgemeine Genetik”, Gustav Fischer Verlag, Stuttgart, 1986).

[0044] Suitable mutations include transitions, transversions, insertions and deletions. Depending on the action of the amino acid exchange on the enzyme activity, one speaks of missense mutations or nonsense mutations. Insertions or deletions of at least one base pair in a gene lead to frame shift mutations, which in turn lead to the incorporation of false amino acids or the premature termination of a translation. If as a result of the mutation a stop codon is formed in the coding region, this also leads to a premature termination of the translation. Deletions of several codons typically lead to a complete disruption of the enzyme activity. Details regarding the production of such mutations belong to the prior art and may be obtained from known textbooks on genetics and molecular biology, such as for example the textbook by Knippers (“Molekulare Genetik”, 6^(th) Edition, Georg Thieme Verlag, Stuttgart, Germany, 1995), that by Winnacker (“Gene und Klone”, VCH Verlagsgesellschaft, Weinheim, Germany, 1990) or that by Hagemann (“Allgemeine Genetik”, Gustav Fischer Verlag, Stuttgart, 1986).

[0045] Suitable mutations in the genes such as for example deletion mutations may be incorporated by gene and/or allele exchange in suitable strains.

[0046] A conventional method is the method of gene exchange by means of a conditionally replicating pSC101 derivate pMAK705 described by Hamilton et al. (Journal of Bacteriology 171, 4617-4622 (1989)). Other methods described in the prior art, such as for example that of Martinez-Morales et al. (Journal of Bacteriology 181, 7143-7148 (1999)) or that of Boyd et al. (Journal of Bacteriology 182, 842-847 (2000)) may likewise be used.

[0047] It is also possible to transfer mutations in the respective genes or mutations relating to the expression of the relevant genes, by conjugation or transduction into various strains.

[0048] Furthermore for the production of L-amino acids, in particular L-threonine, using strains of the family Enterobacteriaceae it may be advantageous in addition to the attenuation of the aceA gene also to enhance one or more enzymes of the known threonine biosynthesis pathway or enzymes of anaplerotic metabolism or enzymes for the production of reduced nicotinamide-adenine-dinucleotide phosphate.

[0049] The term “enhancement” describes in this connection the raising of the intracellular activity of one or more enzymes or proteins in a microorganism that are coded by the corresponding DNA, by for example increasing the number of copies of the gene or genes, using a strong promoter or a gene that codes for a corresponding enzyme or protein having a high activity, and optionally by combining these measures.

[0050] By means of the enhancement measures, in particular overexpression, the activity or concentration of the corresponding protein is in general raised by at least 10%, 25%, 50%, 75%, 100%, 150%, 200%, 300%, 400% or 500%, at most up to 1000% or 2000% referred to that of the wild type protein and/or the activity or concentration of the protein in the initial microorganism.

[0051] Thus, one or more of the genes selected from the following group may for example by simultaneously enhanced, in particular overexpressed: the thrABC operon coding for aspartate kinase, homoserine dehydrogenase, homoserine kinase and threonine synthase (U.S. Pat. No. 4,278,765),

[0052] the pyc gene coding for pyruvate carboxylase (DE-A-19 831 609), the pps gene coding for phosphoenol pyruvate synthase (Molecular and General Genetics 231:332 (1992)),

[0053] the ppc gene coding for phosphoenol pyruvate carboxylase (Gene 31:279-283 (1984)), the genes pntA and pntB coding for transhydrogenase (European Journal of Biochemistry 158:647-653 (1986)),

[0054] the gene rhtB imparting homoserine resistance (EP-A-0 994 190),

[0055] the mqo gene coding for malate:quinone oxidoreductase (DE 100 348 33.5),

[0056] the gene rhtC imparting threonine resistance (EP-A-1 013 765), and

[0057] the thrE gene of Corynebacterium glutamicum coding for threonine export (DE 100 264 94.8).

[0058] The use of endogenous genes is in general preferred. The term “endogenous genes” or “endogenous nucleotide sequences” is understood to mean the genes or nucleotide sequences present in the population of a species.

[0059] Furthermore for the production of L-amino acids, in particular L-threonine, it may be advantageous in addition to the attenuation of the aceA gene also to attenuate, in particular to switch off or reduce the expression of one or more of the genes selected from the following group:

[0060] the tdh gene coding for threonine dehydrogenase (Ravnikar and Somerville, Journal of Bacteriology 169, 4716-4721 (1987)),

[0061] the mdh gene coding for malate dehydrogenase (E.C. 1.1.1.37) (Vogel et al., Archives in Microbiology 149, 36-42 (1987)),

[0062] the gene product of the open reading frame (orf) yjfA (Accession Number AAC77180 of the National Center for Biotechnology Information (NCBI, Bethesda, Md., USA),

[0063] the gene product of the open reading frame (orf) ytfp (Accession Number AAC77179 des National Center for Biotechnology Information (NCBI, Bethesda, Md., USA),

[0064] the pckA gene coding for the enzyme phosphoenol pyruvate carboxykinase (Medina et al. (Journal of Bacteriology 172, 7151-7156 (1990)),

[0065] the poxB gene coding for pyruvate oxidase (Grabau and Cronan (Nucleic Acids Research 14 (13), 5449-5460 (1986)),

[0066] the dgsA gene coding for the regulator of the phosphotransferase system (Hosono et al., Bioscience, Biotechnology and Biochemistry 59, 256-251 (1995) and

[0067] Accession No.: AE000255), and

[0068] the fruR gene coding for the fructose repressor (Jahreis et al., Molecular and General Genetics 226, 332-336 (1991) and Accession No.: AE000118)

[0069] Furthermore for the production of L-amino acids, in particular L-threonine, it may be advantageous in addition to the attenuation of the aceA gene also to switch off undesirable secondary reactions (Nakayama: “Breeding of Amino Acid Producing Microorganisms”, in: Overproduction of Microbial Products, Krumphanzl, Sikyta, Vanek (eds.), Academic Press, London, UK, 1982).

[0070] The microorganisms produced according to the invention may be cultivated in a batch process (batch cultivation), in a fed batch process (feed process) or in a repeated fed batch process (repetitive feed process). A summary of known cultivation methods is described in the textbook by Chmiel (Bioprozesstechnik 1. Einführung in die Bioverfahrenstechnik (Gustav Fischer Verlag, Stuttgart, 1991)) or in the textbook by Storhas (Bioreaktoren and periphere Einrichtungen (Vieweg Verlag, Brunswick/Wiesbaden, 1994)).

[0071] The culture medium to be used must appropriately satisfy the requirements of the respective strains. Descriptions of culture media of various microorganisms are contained in the handbook “Manual of Methods for General Bacteriology” of the American Society for Bacteriology (Washington D.C., USA, 1981).

[0072] As carbon sources, sugars and carbohydrates such as for example glucose, sucrose, lactose, fructose, maltose, molasses, starch and optionally cellulose, oils and fats such as for example soya bean oil, sunflower oil, groundnut oil and coconut oil, fatty acids such as for example palmitic acid, stearic acid and linoleic acid, alcohols such as for example glycerol and ethanol, and organic acids such as for example acetic acid, may be used. These substances may be used individually or as a mixture.

[0073] As nitrogen source, organic nitrogen-containing compounds such as peptones, yeast extract, meat extract, malt extract, maize starch water, soya bean flour and urea or inorganic compounds such as ammonium sulfate, ammonium chloride, ammonium phosphate, ammonium carbonate and ammonium nitrate may be used. The nitrogen sources may be used individually or as a mixture. As phosphorus source, phosphoric acid, potassium dihydrogen phosphate or dipotassium hydrogen phosphate or the corresponding sodium-containing salts may be used. The culture medium must furthermore contain salts of metals, such as for example magnesium sulfate or iron sulfate, that are necessary for growth. Finally, essential growth promoters such as amino acids and vitamins may be used in addition to the aforementioned substances. Apart from these, suitable precursors may be added to the culture medium. The aforementioned starting substances may be added to the culture in the form of a single batch or may be metered in in an appropriate manner during the cultivation.

[0074] In order to regulate the pH of the culture basic compounds such as sodium hydroxide, potassium hydroxide, ammonia or ammonia water, or acidic compounds such as phosphoric acid or sulfuric acid are used as appropriate. In order to control foam formation antifoaming agents such as for example fatty acid polyglycol esters may be used. In order to maintain the stability of plasmids, suitable selectively acting substances, for example antibiotics, may be added to the medium. In order to maintain aerobic conditions, oxygen or oxygen-containing gas mixtures such as for example air are fed into the culture. The temperature of the culture is normally 25° C. to 45° C., and preferably 30° C. to 40° C. Cultivation is continued until a maximum amount of L-amino acids (or L-threonine) has been formed. This target is normally achieved within 10 hours to 160 hours.

[0075] The L-amino acids may be analyzed by anion exchange chromatography followed by ninhydrin derivation, as described by Spackman et al. (Analytical Chemistry, 30, (1958), 1190), or by reversed phase HPLC, as described by Lindroth et al. (Analytical Chemistry (1979) 51: 1167-1174).

[0076] The process according to the invention can be used for the enzymatic production of L-amino acids, such as for example L-threonine, L-isoleucine, L-valine, L-methionine, L-homoserine and L-lysine, in particular L-threonine.

[0077] A pure culture of the Escherichia coli K-12 strain DH5a/pMAK705 was filed as DSM 13720 on Sep. 8, 2000 at the German Collection for Microorganisms and Cell Cultures (DSMZ, Brunswick, Germany) according to the Budapest Convention.

[0078] The present invention is described in more detail hereinafter with the aid of examples of implementation.

[0079] The isolation of plasmid DNA from Escherichia coli as well as all techniques for the restriction, Klenow treatment and alkaline phosphatase treatment are carried out according to Sambrook et al. (Molecular Cloning—A Laboratory Manual (1989) Cold Spring Harbor Laboratory Press). The transformation of Escherichia coli is, unless otherwise described, carried out according to Chung et al. (Proceedings of the National Academy of Sciences of the United States of America, USA (1989) 86: 2172-2175).

[0080] The incubation temperature in the production of strains and transformants is 37° C. In the gene exchange process according to Hamilton et al., temperatures of 30° C. and 44° C. are used.

EXAMPLES

[0081] Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. cl Example 1

Construction of the Deletion Mutation of the aceA Gene

[0082] Parts of the gene regions lying upstream and downstream of the aceA gene are amplified from Escherichia coli K12 using the polymerase chain reaction (PCR) as well as synthetic oligonucleotides. Starting from the nucleotide sequence of the aceBAK operon in E. coli K12 MG1655 DNA (SEQ ID No. 1) the following PCR primers are synthesized (MWG Biotech, Ebersberg, Germany): aceA′5′-1: 5′-ATGCTTACTCACGCCTGTTG-3′ (SEQ ID No.3) aceA′5′-2: 5′-CATGTGCAGATGCTCCATAG-3′ (SEQ ID No.4) aceA′3′-1: 5′-CAACAACAACCGTTGCTGAC-3′ (SEQ ID No.5) aceA′3′-2: 5′-CAGTTCGTTCGCCACCTGTA-3′ (SEQ ID No.6)

[0083] The chromosomal E. coli K12 MG1655 DNA used for the PCR is isolated according to the manufacturer's instructions using “Qiagen Genomic-tips 100/G” (QIAGEN, Hilden, Germany). A ca. 700 bp large DNA fragment from the region lying upstream of the aceA gene (designated ′aceB) and a ca. 800 bp large DNA fragment from the region lying downstream of the aceA gene (designated aceK') can be amplified with the specific primers under standard PCR conditions (Innis et al. (1990) PCR Protocols. A guide to methods and applications, Academic Press) with Taq DNA polymerase (Gibco-BRL, Eggenstein, Germany). The PCR products are ligated according to the manufacturer's instructions in each case with the vector pCR2. lTOPO (TOPO TA Cloning Kit, Invitrogen, Groningen, Netherlands) and transformed in the E. coli strain TOP 10F′. The selection of plasmid-carrying cells is carried out on LB agar to which 50 μg/ml of ampicillin has been added.

[0084] After the plasmid DNA isolation the vector pCR2.1 TOPO′ aceB is cleaved with the restriction enzymes EcoRV and SpeI, and the ′aceB fragment after separation in 0.8% agarose gel is isolated using the QIAquick Gel Extraction Kit (QIAGEN, Hilden, Germany). After the plasmid DNA isolation the vector pCR2.1TOPOaceK' is cleaved with the enzymes Ec11361I and SpeI and ligated with the isolated ′aceB fragment. The E. coli strain DH5a is transformed with the ligation batch and plasmid-carrying cells are selected on LB agar to which 50 μg/ml of ampicillin has been added. After the plasmid DNA isolation, those plasmids in which the mutagenic DNA sequence shown in SEQ ID No. 7 is present in cloned form are detected by control cleavage with the enzymes HindIII and XbaI. One of the plasmids is designated pCR2.1TOPOΔaceA.

Example 2 Construction of the Exchange Vector pMAK705ΔaceA

[0085] The aceBAK allele described in Example 1 is isolated from the vector pCR2.1TOPOΔaceA after restriction with the enzymes HindIIi and XbaI and separation in 0.8% agarose gel, and is ligated with the plasmid pMAK705 (Hamilton et al. (1989) Journal of Bacteriology 171, 4617-4622), that had been digested with the enzymes HindIII and XbaI. The ligation batch is transformed in DH5α and plasmid-carrying cells are selected on LB agar to which 20 μg/ml of chloramphenicol have been added. The successful cloning is detected after plasmid DNA isolation and cleavage with the enzymes BamHI, KpnI, SphI, SpeI and PstI. The resultant exchange vector pMAK705ΔaceA (=pMAK705deltaaceA) is shown in FIG. 1.

Example 3 Site-specific Mutagenesis of the aceA Gene in the E. coli Strain MG442

[0086] The E. coli

[0087] strain MG442 producing L-threonine is described in patent specification U.S. Pat. No. 4,278,765 and is filed as CMIM B-1628 at the Russian National Collection for Industrial Microorganisms (VKPM, Moscow, Russia).

[0088] For the exchange of the chromosomal aceA gene by the plasmid-coded deletion construct, MG442 is transformed with the plasmid pMAK705AaceA. The gene exchange is carried out by the selection process described by Hamilton et al. (1989) Journal of Bacteriology 171, 4617-4622) and is verified by standard PCR methods (Innis et al. (1990) PCR Protocols. A guide to methods and applications, Academic Press) with the following oligonucleotide primers: aceA′5′-1: 5′-ATGCTTACTCACGCCTGTTG-3′ (SEQ ID No.3) aceA′3′-2: 5′-CAGTTCGTTCGCCACCTGTA-3′ (SEQ ID No.6)

Example 4 Production of L-threonine Using the Strain MG442ΔaceA

[0089] MG442AaceA is cultivated on minimal medium having the following composition: 3.5 g/l Na₂HPO₄.2H₂O, 1.5 g/l KH₂PO₄, 1 g/l NH₄Cl, 0.1 g/l MgSO₄.7H₂O, 2 g/l glucose and 20 g/l agar. The formation of L-threonine is checked in batch cultures of 10 ml that are contained in 100 ml Erlenmeyer flasks. For this, 10 ml of preculture medium of the following composition: 2 g/l yeast extract, 10 g/l (NH₄)₂SO₄, 1 g/l KH₂PO₄, 0.5 g/l MgSO₄.7H₂O, 15 g/l CaCO₃, 20 g/l glucose are inoculated and incubated for 16 hours at 37° C. and 180 rpm in an ESR incubator from Küthner AG (Birsfelden, Switzerland). 250 μl of this preculture are reinoculated in 10 ml of production medium (25 g/l (NH₄)₂SO₄, 2 g/l KH₂PO₄, 1 g/l MgSO₄.7H₂O, 0.03 g/l FeSO₄.7H₂O, 0.018 g/l MnSO₄.1H₂O, 30 g/l CaCO₃ and 20 g/l glucose) and incubated for 48 hours at 37° C. After incubation the optical density (OD) of the culture suspension is measured with an LP2W photometer from the Dr. Lange company (Dusseldorf, Germany) at a measurement wavelength of 660 nm.

[0090] The concentration of formed L-threonine is then determined in the sterile-filtered culture supernatant using an amino acid analyzer from Eppendorf-BioTronik (Hamburg, Germany) by ion exchange chromatography and post-column reaction with ninhydrin detection.

[0091] The result of the test is given in Table 1. TABLE 1 OD Strain (660 nm) L-threonine g/l MG442 6.0 1.5 MG442ΔaceA 6.2 1.9

[0092] The publications cited herein are incorporated herein by reference.

[0093] Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

[0094] This application is based on German Patent Application Serial No. 101 16 518.8, filed on Apr. 3, 200 1, and incorporated herein by reference in its entirety.

1 7 1 4855 DNA Escherichia coli CDS (1632)..(2936) 1 atgactgaac aggcaacaac aaccgatgaa ctggctttca caaggccgta tggcgagcag 60 gagaagcaaa ttcttactgc cgaagcggta gaatttctga ctgagctggt gacgcatttt 120 acgccacaac gcaataaact tctggcagcg cgcattcagc agcagcaaga tattgataac 180 ggaacgttgc ctgattttat ttcggaaaca gcttccattc gcgatgctga ttggaaaatt 240 cgcgggattc ctgcggactt agaagaccgc cgcgtagaga taactggccc ggtagagcgc 300 aagatggtga tcaacgcgct caacgccaat gtgaaagtct ttatggccga tttcgaagat 360 tcactggcac cagactggaa caaagtgatc gacgggcaaa ttaacctgcg tgatgcggtt 420 aacggcacca tcagttacac caatgaagca ggcaaaattt accagctcaa gcccaatcca 480 gcggttttga tttgtcgggt acgcggtctg cacttgccgg aaaaacatgt cacctggcgt 540 ggtgaggcaa tccccggcag cctgtttgat tttgcgctct atttcttcca caactatcag 600 gcactgttgg caaagggcag tggtccctat ttctatctgc cgaaaaccca gtcctggcag 660 gaagcggcct ggtggagcga agtcttcagc tatgcagaag atcgctttaa tctgccgcgc 720 ggcaccatca aggcgacgtt gctgattgaa acgctgcccg ccgtgttcca gatggatgaa 780 atccttcacg cgctgcgtga ccatattgtt ggtctgaact gcggtcgttg ggattacatc 840 ttcagctata tcaaaacgtt gaaaaactat cccgatcgcg tcctgccaga cagacaggca 900 gtgacgatgg ataaaccatt cctgaatgct tactcacgcc tgttgattaa aacctgccat 960 aaacgcggtg cttttgcgat gggcggcatg gcggcgttta ttccgagcaa agatgaagag 1020 cacaataacc aggtgctcaa caaagtaaaa gcggataaat cgctggaagc caataacggt 1080 cacgatggca catggatcgc tcacccaggc cttgcggaca cggcaatggc ggtattcaac 1140 gacattctcg gctcccgtaa aaatcagctt gaagtgatgc gcgaacaaga cgcgccgatt 1200 actgccgatc agctgctggc accttgtgat ggtgaacgca ccgaagaagg tatgcgcgcc 1260 aacattcgcg tggctgtgca gtacatcgaa gcgtggatct ctggcaacgg ctgtgtgccg 1320 atttatggcc tgatggaaga tgcggcgacg gctgaaattt cccgtacctc gatctggcag 1380 tggatccatc atcaaaaaac gttgagcaat ggcaaaccgg tgaccaaagc cttgttccgc 1440 cagatgctgg gcgaagagat gaaagtcatt gccagcgaac tgggcgaaga acgtttctcc 1500 caggggcgtt ttgacgatgc cgcacgcttg atggaacaga tcaccacttc cgatgagtta 1560 attgatttcc tgaccctgcc aggctaccgc ctgttagcgt aaaccaccac ataactatgg 1620 agcatctgca c atg aaa acc cgt aca caa caa att gaa gaa tta cag aaa 1670 Met Lys Thr Arg Thr Gln Gln Ile Glu Glu Leu Gln Lys 1 5 10 gag tgg act caa ccg cgt tgg gaa ggc att act cgc cca tac agt gcg 1718 Glu Trp Thr Gln Pro Arg Trp Glu Gly Ile Thr Arg Pro Tyr Ser Ala 15 20 25 gaa gat gtg gtg aaa tta cgc ggt tca gtc aat cct gaa tgc acg ctg 1766 Glu Asp Val Val Lys Leu Arg Gly Ser Val Asn Pro Glu Cys Thr Leu 30 35 40 45 gcg caa ctg ggc gca gcg aaa atg tgg cgt ctg ctg cac ggt gag tcg 1814 Ala Gln Leu Gly Ala Ala Lys Met Trp Arg Leu Leu His Gly Glu Ser 50 55 60 aaa aaa ggc tac atc aac agc ctc ggc gca ctg act ggc ggt cag gcg 1862 Lys Lys Gly Tyr Ile Asn Ser Leu Gly Ala Leu Thr Gly Gly Gln Ala 65 70 75 ctg caa cag gcg aaa gcg ggt att gaa gca gtc tat ctg tcg gga tgg 1910 Leu Gln Gln Ala Lys Ala Gly Ile Glu Ala Val Tyr Leu Ser Gly Trp 80 85 90 cag gta gcg gcg gac gct aac ctg gcg gcc agc atg tat ccg gat cag 1958 Gln Val Ala Ala Asp Ala Asn Leu Ala Ala Ser Met Tyr Pro Asp Gln 95 100 105 tcg ctc tat ccg gca aac tcg gtg cca gct gtg gtg gag cgg atc aac 2006 Ser Leu Tyr Pro Ala Asn Ser Val Pro Ala Val Val Glu Arg Ile Asn 110 115 120 125 aac acc ttc cgt cgt gcc gat cag atc caa tgg tcc gcg ggc att gag 2054 Asn Thr Phe Arg Arg Ala Asp Gln Ile Gln Trp Ser Ala Gly Ile Glu 130 135 140 ccg ggc gat ccg cgc tat gtc gat tac ttc ctg ccg atc gtt gcc gat 2102 Pro Gly Asp Pro Arg Tyr Val Asp Tyr Phe Leu Pro Ile Val Ala Asp 145 150 155 gcg gaa gcc ggt ttt ggc ggt gtc ctg aat gcc ttt gaa ctg atg aaa 2150 Ala Glu Ala Gly Phe Gly Gly Val Leu Asn Ala Phe Glu Leu Met Lys 160 165 170 gcg atg att gaa gcc ggt gca gcg gca gtt cac ttc gaa gat cag ctg 2198 Ala Met Ile Glu Ala Gly Ala Ala Ala Val His Phe Glu Asp Gln Leu 175 180 185 gcg tca gtg aag aaa tgc ggt cac atg ggc ggc aaa gtt tta gtg cca 2246 Ala Ser Val Lys Lys Cys Gly His Met Gly Gly Lys Val Leu Val Pro 190 195 200 205 act cag gaa gct att cag aaa ctg gtc gcg gcg cgt ctg gca gct gac 2294 Thr Gln Glu Ala Ile Gln Lys Leu Val Ala Ala Arg Leu Ala Ala Asp 210 215 220 gtg acg ggc gtt cca acc ctg ctg gtt gcc cgt acc gat gct gat gcg 2342 Val Thr Gly Val Pro Thr Leu Leu Val Ala Arg Thr Asp Ala Asp Ala 225 230 235 gcg gat ctg atc acc tcc gat tgc gac ccg tat gac agc gaa ttt att 2390 Ala Asp Leu Ile Thr Ser Asp Cys Asp Pro Tyr Asp Ser Glu Phe Ile 240 245 250 acc ggc gag cgt acc agt gaa ggc ttc ttc cgt act cat gcg ggc att 2438 Thr Gly Glu Arg Thr Ser Glu Gly Phe Phe Arg Thr His Ala Gly Ile 255 260 265 gag caa gcg atc agc cgt ggc ctg gcg tat gcg cca tat gct gac ctg 2486 Glu Gln Ala Ile Ser Arg Gly Leu Ala Tyr Ala Pro Tyr Ala Asp Leu 270 275 280 285 gtc tgg tgt gaa acc tcc acg ccg gat ctg gaa ctg gcg cgt cgc ttt 2534 Val Trp Cys Glu Thr Ser Thr Pro Asp Leu Glu Leu Ala Arg Arg Phe 290 295 300 gca caa gct atc cac gcg aaa tat ccg ggc aaa ctg ctg gct tat aac 2582 Ala Gln Ala Ile His Ala Lys Tyr Pro Gly Lys Leu Leu Ala Tyr Asn 305 310 315 tgc tcg ccg tcg ttc aac tgg cag aaa aac ctc gac gac aaa act att 2630 Cys Ser Pro Ser Phe Asn Trp Gln Lys Asn Leu Asp Asp Lys Thr Ile 320 325 330 gcc agc ttc cag cag cag ctg tcg gat atg ggc tac aag ttc cag ttc 2678 Ala Ser Phe Gln Gln Gln Leu Ser Asp Met Gly Tyr Lys Phe Gln Phe 335 340 345 atc acc ctg gca ggt atc cac agc atg tgg ttc aac atg ttt gac ctg 2726 Ile Thr Leu Ala Gly Ile His Ser Met Trp Phe Asn Met Phe Asp Leu 350 355 360 365 gca aac gcc tat gcc cag ggc gag ggt atg aag cac tac gtt gag aaa 2774 Ala Asn Ala Tyr Ala Gln Gly Glu Gly Met Lys His Tyr Val Glu Lys 370 375 380 gtg cag cag ccg gaa ttt gcc gcc gcg aaa gat ggc tat acc ttc gta 2822 Val Gln Gln Pro Glu Phe Ala Ala Ala Lys Asp Gly Tyr Thr Phe Val 385 390 395 tct cac cag cag gaa gtg ggt aca ggt tac ttc gat aaa gtg acg act 2870 Ser His Gln Gln Glu Val Gly Thr Gly Tyr Phe Asp Lys Val Thr Thr 400 405 410 att att cag ggc ggc acg tct tca gtc acc gcg ctg acc ggc tcc act 2918 Ile Ile Gln Gly Gly Thr Ser Ser Val Thr Ala Leu Thr Gly Ser Thr 415 420 425 gaa gaa tcg cag ttc taa gcaacaacaa ccgttgctga ctgtaggccg 2966 Glu Glu Ser Gln Phe 430 gataaggcgt tcacgccgca tccggcaatc ggtgcacgat gcctgatgcg acgcttgcgc 3026 gtcttatcat gcctacagcc gttgccgaac gtaggctgga taaggcgttt acgccgcatc 3086 cggcaattct ctgctcctga tgagggcgct aaatgccgcg tggcctggaa ttattgattg 3146 ctcaaaccat tttgcaaggc ttcgatgctc agtatggtcg attcctcgaa gtgacctccg 3206 gtgcgcagca gcgtttcgaa caggccgact ggcatgctgt ccagcaggcg atgaaaaacc 3266 gtatccatct ttacgatcat cacgttggtc tggtcgtgga gcaactgcgc tgcattacta 3326 acggccaaag tacggacgcg gcatttttac tacgtgttaa agagcattac acccggctgt 3386 tgccggatta cccgcgcttc gagattgcgg agagcttttt taactccgtg tactgtcggt 3446 tatttgacca ccgctcgctt actcccgagc ggctttttat ctttagctct cagccagagc 3506 gccgctttcg taccattccc cgcccgctgg cgaaagactt tcaccccgat cacggctggg 3566 aatctctact gatgcgcgtt atcagcgacc taccgctgcg cctgcgctgg cagaataaaa 3626 gccgtgacat ccattacatt attcgccatc tgacggaaac gctggggaca gacaacctcg 3686 cggaaagtca tttacaggtg gcgaacgaac tgttttaccg caataaagcc gcctggctgg 3746 taggcaaact gatcacacct tccggcacat tgccattttt gctgccgatc caccagacgg 3806 acgacggcga gttatttatt gatacctgcc tgacgacgac cgccgaagcg agcattgttt 3866 ttggctttgc gcgttcttat tttatggttt atgcgccgct gcccgcagcg ctggtcgagt 3926 ggctacggga aattctgcca ggtaaaacca ccgctgaatt gtatatggct atcggctgcc 3986 agaagcacgc caaaaccgaa agctaccgcg aatatctcgt ttatctacag ggctgtaatg 4046 agcagttcat tgaagcgccg ggtattcgtg gaatggtgat gttggtgttt acgctgccgg 4106 gctttgatcg ggtattcaaa gtcatcaaag acaggttcgc gccgcagaaa gagatgtctg 4166 ccgctcacgt tcgtgcctgc tatcaactgg tgaaagagca cgatcgcgtg ggccgaatgg 4226 cggacaccca ggagtttgaa aactttgtgc tggagaagcg gcatatttcc ccggcattaa 4286 tggaattact gcttcaggaa gcagcggaaa aaatcaccga tctcggcgaa caaattgtga 4346 ttcgccatct ttatattgag cggcggatgg tgccgctcaa tatctggctg gaacaagtgg 4406 aaggtcagca gttgcgcgac gccattgaag aatacggtaa cgctattcgc cagcttgccg 4466 ctgctaacat tttccctggc gacatgctgt ttaaaaactt cggtgtcacc cgtcacgggc 4526 gtgtggtttt ttatgattac gatgaaattt gctacatgac ggaagtgaat ttccgcgaca 4586 tcccgccgcc gcgctatccg gaagacgaac ttgccagcga accgtggtac agcgtctcgc 4646 cgggcgatgt tttcccggaa gagtttcgcc actggctatg cgccgacccg cgtattggtc 4706 cgctgtttga agagatgcac gccgacctgt tccgcgctga ttactggcgc gcactacaaa 4766 accgcatacg tgaagggcat gtggaagatg tttatgcgta tcggcgcagg caaagattta 4826 gcgtacggta tggggagatg cttttttga 4855 2 434 PRT Escherichia coli 2 Met Lys Thr Arg Thr Gln Gln Ile Glu Glu Leu Gln Lys Glu Trp Thr 1 5 10 15 Gln Pro Arg Trp Glu Gly Ile Thr Arg Pro Tyr Ser Ala Glu Asp Val 20 25 30 Val Lys Leu Arg Gly Ser Val Asn Pro Glu Cys Thr Leu Ala Gln Leu 35 40 45 Gly Ala Ala Lys Met Trp Arg Leu Leu His Gly Glu Ser Lys Lys Gly 50 55 60 Tyr Ile Asn Ser Leu Gly Ala Leu Thr Gly Gly Gln Ala Leu Gln Gln 65 70 75 80 Ala Lys Ala Gly Ile Glu Ala Val Tyr Leu Ser Gly Trp Gln Val Ala 85 90 95 Ala Asp Ala Asn Leu Ala Ala Ser Met Tyr Pro Asp Gln Ser Leu Tyr 100 105 110 Pro Ala Asn Ser Val Pro Ala Val Val Glu Arg Ile Asn Asn Thr Phe 115 120 125 Arg Arg Ala Asp Gln Ile Gln Trp Ser Ala Gly Ile Glu Pro Gly Asp 130 135 140 Pro Arg Tyr Val Asp Tyr Phe Leu Pro Ile Val Ala Asp Ala Glu Ala 145 150 155 160 Gly Phe Gly Gly Val Leu Asn Ala Phe Glu Leu Met Lys Ala Met Ile 165 170 175 Glu Ala Gly Ala Ala Ala Val His Phe Glu Asp Gln Leu Ala Ser Val 180 185 190 Lys Lys Cys Gly His Met Gly Gly Lys Val Leu Val Pro Thr Gln Glu 195 200 205 Ala Ile Gln Lys Leu Val Ala Ala Arg Leu Ala Ala Asp Val Thr Gly 210 215 220 Val Pro Thr Leu Leu Val Ala Arg Thr Asp Ala Asp Ala Ala Asp Leu 225 230 235 240 Ile Thr Ser Asp Cys Asp Pro Tyr Asp Ser Glu Phe Ile Thr Gly Glu 245 250 255 Arg Thr Ser Glu Gly Phe Phe Arg Thr His Ala Gly Ile Glu Gln Ala 260 265 270 Ile Ser Arg Gly Leu Ala Tyr Ala Pro Tyr Ala Asp Leu Val Trp Cys 275 280 285 Glu Thr Ser Thr Pro Asp Leu Glu Leu Ala Arg Arg Phe Ala Gln Ala 290 295 300 Ile His Ala Lys Tyr Pro Gly Lys Leu Leu Ala Tyr Asn Cys Ser Pro 305 310 315 320 Ser Phe Asn Trp Gln Lys Asn Leu Asp Asp Lys Thr Ile Ala Ser Phe 325 330 335 Gln Gln Gln Leu Ser Asp Met Gly Tyr Lys Phe Gln Phe Ile Thr Leu 340 345 350 Ala Gly Ile His Ser Met Trp Phe Asn Met Phe Asp Leu Ala Asn Ala 355 360 365 Tyr Ala Gln Gly Glu Gly Met Lys His Tyr Val Glu Lys Val Gln Gln 370 375 380 Pro Glu Phe Ala Ala Ala Lys Asp Gly Tyr Thr Phe Val Ser His Gln 385 390 395 400 Gln Glu Val Gly Thr Gly Tyr Phe Asp Lys Val Thr Thr Ile Ile Gln 405 410 415 Gly Gly Thr Ser Ser Val Thr Ala Leu Thr Gly Ser Thr Glu Glu Ser 420 425 430 Gln Phe 3 20 DNA Artificial sequence Synthetic DNA 3 atgcttactc acgcctgttg 20 4 20 DNA Artificial sequence Synthetic DNA 4 catgtgcaga tgctccatag 20 5 20 DNA Artificial sequence Synthetic DNA 5 caacaacaac cgttgctgac 20 6 20 DNA Artificial sequence Synthetic DNA 6 cagttcgttc gccacctgta 20 7 1643 DNA Escherichia coli misc_feature (1)..(33) Technical DNA/ remainder polylinker sequence 7 agcttggtac cgagatctgc agaattcgcc cttatgctta ctcacgcctg ttgattaaaa 60 cctgccataa acgcggtgct tttgcgatgg gcggcatggc ggcgtttatt ccgagcaaag 120 atgaagagca caataaccag gtgctcaaca aagtaaaagc ggataaatcg ctggaagcca 180 ataacggtca cgatggcaca tggatcgctc acccaggcct tgcggacacg gcaatggcgg 240 tattcaacga cattctcggc tcccgtaaaa atcagcttga agtgatgcgc gaacaagacg 300 cgccgattac tgccgatcag ctgctggcac cttgtgatgg tgaacgcacc gaagaaggta 360 tgcgcgccaa cattcgcgtg gctgtgcagt acatcgaagc gtggatctct ggcaacggct 420 gtgtgccgat ttatggcctg atggaagatg cggcgacggc tgaaatttcc cgtacctcga 480 tctggcagtg gatccatcat caaaaaacgt tgagcaatgg caaaccggtg accaaagcct 540 tgttccgcca gatgctgggc gaagagatga aagtcattgc cagcgaactg ggcgaagaac 600 gtttctccca ggggcgtttt gacgatgccg cacgcttgat ggaacagatc accacttccg 660 atgagttaat tgatttcctg accctgccag gctaccgcct gttagcgtaa accaccacat 720 aactatggag catctgcaca tgaagggcga attccagcac actggcggcc gttactagta 780 acggccgcca gtgtgctgga attcgccctt caacaacaac cgttgctgac tgtaggccgg 840 ataaggcgtt cacgccgcat ccggcaatcg gtgcacgatg cctgatgcga cgcttgcgcg 900 tcttatcatg cctacagccg ttgccgaacg taggctggat aaggcgttta cgccgcatcc 960 ggcaattctc tgctcctgat gagggcgcta aatgccgcgt ggcctggaat tattgattgc 1020 tcaaaccatt ttgcaaggct tcgatgctca gtatggtcga ttcctcgaag tgacctccgg 1080 tgcgcagcag cgtttcgaac aggccgactg gcatgctgtc cagcaggcga tgaaaaaccg 1140 tatccatctt tacgatcatc acgttggtct ggtcgtggag caactgcgct gcattactaa 1200 cggccaaagt acggacgcgg catttttact acgtgttaaa gagcattaca cccggctgtt 1260 gccggattac ccgcgcttcg agattgcgga gagctttttt aactccgtgt actgtcggtt 1320 atttgaccac cgctcgctta ctcccgagcg gctttttatc tttagctctc agccagagcg 1380 ccgctttcgt accattcccc gcccgctggc gaaagacttt caccccgatc acggctggga 1440 atctctactg atgcgcgtta tcagcgacct accgctgcgc ctgcgctggc agaataaaag 1500 ccgtgacatc cattacatta ttcgccatct gacggaaacg ctggggacag acaacctcgc 1560 ggaaagtcat ttacaggtgg cgaacgaact gaagggcgaa ttctgcagat atccatcaca 1620 ctggcggccg ctcgagcatg cat 1643 

What is claimed is:
 1. A process for the production of an L-amino acid, comprising: (a) fermenting a microorganism of the family Enterobacteriaceae which produces the desired L-amino acid, in which the aceA gene or nucleotide sequences coding therefor are attenuated, in a medium; (b) enriching the medium or the cells of the microorganism in the L-amino acid, and (c) isolating the L-amino acid.
 2. The process of claim 1, wherein the L-amino acid is L-threonine.
 3. The process of claim 1, wherein the aceA gene or nucleotide sequences coding therefor are switched off.
 4. The process of claim 1, wherein constituents of the fermentation medium and/or the biomass in its entirety or portions thereof remain in the isolated L-amino acid.
 5. The process of claim 1, wherein one or more genes in the biosynthesis pathway of the L-amino acid are enhanced in the microorganism.
 6. The process of claim 1, wherein the metabolic pathways that reduce the formation of the L-amino acid are at least partially switched off in the microorganism.
 7. The process of claim 1, wherein the expression of the aceA gene or nucleotide sequences coding therefor is attenuated.
 8. The process of claim 1, wherein the expression of the aceA gene or nucleotide sequences coding therefor is switched off.
 9. The process of claim 1, wherein the regulatory and/or catalytic properties of the polypeptide for which the aceA gene encodes are reduced.
 10. The process of claim 1, wherein in the microorganism one or more of the genes selected from the following group is enhanced: the thrABC operon coding for aspartate kinase, homoserine dehydrogenase, homoserine kinase and threonine synthase, the pyc gene coding for pyruvate carboxylase, the pps gene coding for phosphoenol pyruvate synthase, the ppc gene coding for phosphoenol pyruvate carboxylase, the pntA and pntB genes coding for transhydrogenase, the rhtB gene imparting homoserine resistance, the mqo gene coding for malate:quinone oxidoreductase, the rhtC gene imparting threonine resistance, and the thrE gene coding for threonine export.
 11. The process of claim 1, wherein in the microorganism one or more of the genes selected from the following group is overexpressed: the thrABC operon coding for aspartate kinase, homoserine dehydrogenase, homoserine kinase and threonine synthase, the pyc gene coding for pyruvate carboxylase, the pps gene coding for phosphoenol pyruvate synthase, the ppc gene coding for phosphoenol pyruvate carboxylase, the pntA and pntB genes coding for transhydrogenase, the rhtB gene imparting homoserine resistance, the mqo gene coding for malate:quinone oxidoreductase, the rhtC gene imparting threonine resistance, and the thrE gene coding for threonine export.
 12. The process of claim 1, wherein in the microorganism one or more of the genes selected from the following group is attenuated: the tdh gene coding for threonine dehydrogenase, the mdh gene coding for malate dehydrogenase, the gene product of the open reading frame (orf) yjfA, the gene product of the open reading frame (orf) ytfp, the pckA gene coding for phosphoenol pyruvate carboxykinase, the poxB gene coding for pyruvate oxidase, the dgsA gene coding for the regulator of the phosphotransferase system, and the fruR gene coding for the fructose repressor.
 13. The process of claim 1, wherein in the microorganism one or more of the genes selected from the following group is switched off: the tdh gene coding for threonine dehydrogenase, the mdh gene coding for malate dehydrogenase, the gene product of the open reading frame (orf) yjfA, the gene product of the open reading frame (orf) ytfp, the pckA gene coding for phosphoenol pyruvate carboxykinase, the poxB gene coding for pyruvate oxidase, the dgsA gene coding for the regulator of the phosphotransferase system, and the fruR gene coding for the fructose repressor.
 14. The process of claim 1, wherein in the microorganism the expression of one or more of the genes selected from the following group is reduced: the tdh gene coding for threonine dehydrogenase, the mdh gene coding for malate dehydrogenase, the gene product of the open reading frame (orf) yjfA, the gene product of the open reading frame (orf) ytfp, the pckA gene coding for phosphoenol pyruvate carboxykinase, the poxB gene coding for pyruvate oxidase, the dgsA gene coding for the regulator of the phosphotransferase system, and the fruR gene coding for the fructose repressor.
 15. The process of claim 1, wherein the microorganism belongs to the genus Escherichia.
 16. The process of claim 1, wherein the microorganism belongs to the genus Erwinia.
 17. The process of claim 1, wherein the microorganism belongs to the genus Providencia.
 18. The process of claim 1, wherein the microorganism belongs to the genus Serratia.
 19. The process of claim 1, wherein the microorganism is an E. coli.
 20. The process of claim 1, wherein the microorganism is an Enterobacteriaceae selected from the group consisting of Escherichia coli MG442ΔaceA, Escherichia coli TF427, Escherichia coli, Escherichia coli KY10935, Escherichia coli VNIlgenetika MG442, Escherichia coli VNIIgenetika M1, Escherichia coli VNIIgenetika 472T23, Escherichia coli BKIIM B-3996, Escherichia coli kat 13, Escherichia coli KCCM-10132, Serratia marcescens HNr21, Serratia marcescens, and Serratia marcescens T2000.
 21. The process of claim 1, wherein the L-amino acid is selected from the group consisting of L-asparagine, L-serine, L-glutamate, L-glycine, L-alanine, L-cysteine, L-valine, L-methionine, L-isoleucine, L-leucine, L-tyrosine, L-phenylalanine, L-histidine, L-lysine, L-tryptophan, and L-arginine. 